During World War I, Barclay was awarded a prize by the Committee on National Preparedness in 1917 for his poster "Fill the Breach." The next year, he designed naval camouflage under the direction of William Mackay, Chief of the New York District Emergency Fleet Corporation.[2][3]

In 1930, the General Motors Corporation selected Barclay's "Fisher Body Girl" for a series of advertisements, and she quickly became as popular as "The Gibson Girl" and "The Christy Girl". He used his second wife Helene, 19 years old, as the model for the iconic Fisher Autobody image. Helene later appeared in magazine advertisements and was so well published with her languid body plastered across the country on billboards, that she was recognized wherever she went. He also illustrated advertisements for A&P, Eaton Paper Company, Elgin Watches, Humming Bird Hosiery, and Lever Brothers, amongst others.[2] He also illustrated advertisements for Whitman's Chocolates, Texaco, and Camel and Chesterfield brand cigarettes. Because Barclay was known for his illustrations of "striking women", he earned a judging position at the Miss America 1935 pageant.[1]

Barclay did not limit himself to painting. In the late 1930s, he set up a small company to reproduce jewelry and fabricate utilitarian figures for ashtrays, bookends, desk sets, lamps, and other articles for home and office use. These products were fabricated out of cast grey metal with a thick bronze plate finish and they retailed for just a few dollars. The company, which he named the McClelland Barclay Arts Products Corporation, made him little money.[2]

In 1944, a year after his death, Barclay was awarded the Art Directors Club Medal, "in recognition of his long and distinguished record in editorial illustration and advertising art and in honor of his devotion and meritorious service to his country as a commissioned officer of the United States Navy." He was also posthumously inducted in the Society of Illustrators' Hall of Fame.[1]

In June 1938, he was appointed Assistant Naval Constructor with the US Naval Reserve. In mid-1940, Barclay prepared experimental camouflage designs for Navy combat aircraft, but evaluation tests revealed that pattern camouflage was of little use for aircraft. Following the Japanese attack on Pearl Harbor, Barclay completed the first of many recruiting posters for the Navy.[1]

Navy Relief show poster by Barclay (1943).

On October 19, 1940, Barclay reported for active duty. He served in the New York Recruiting office, illustrating posters for the next two and a half years. These images would become some of the most recognizable recruiting images of World War II.[1] Barclay was determined to be a front-line combat artist. In March 1943, he told the San Francisco Examiner, "A camera cannot catch the human element of a fight, the sweat and blood and courage our boys expend every time they face the enemy." In 1941 he volunteered for this position, but was rejected. Eventually he would serve in both the Atlantic and Pacific theaters on the USS Arkansas(BB-33), USS Pennsylvania(BB-38), USS Honolulu(CL-48), and the USS Maryland(BB-46).[4]

Promoted to Lt. Commander, Barclay worked on further assignments until July 18, 1943, when he was reported as missing in action. The USS LST-342 he was aboard was torpedoed in the Solomon Islands.[2] On board, sketching and taking photographs at the time, Barclay's body was not recovered.[1]

Barclay's first wife was Nan McClelland, his niece who was 8 years his senior. McClellan did not smoke or drink and boxed to keep in shape; Nan drank, smoked and loved parties. They grew apart and divorced in February 1930. Barclay then became engaged to his second wife, Helene Haskins. She was 20 and he was 39 – they later divorced. After his divorce with Helene, Barclay was briefly engaged to Virginia Moore, a 22-year old model, in 1937.[5][6]